Type III Secretion System Genotypes and Biofilm-Forming Capacity of Pseudomonas aeruginosa Bloodstream Isolates: An Exploratory Analysis of the exoU–Fluoroquinolone Association

The pathogenicity of Pseudomonas aeruginosa in bloodstream infection is shaped by biofilm formation and the type III secretion system (T3SS). We characterised the biofilm-associated and T3SS genotypes of 100 non-duplicate bloodstream isolates and tested whether either trait co-segregates with antimicrobial resistance. Biofilm was assessed by Congo red agar (CRA) and the quantitative microtiter plate (MTP) assay; biofilm-associated (pslA, pelA, ppyR) and T3SS effector (exoS, exoY, exoT, exoU) genes by conventional PCR; and susceptibility by an automated system interpreted with EUCAST criteria. Biofilm was detected in 84% of isolates by MTP and 25% by CRA (Cohen’s κ = 0.16). pslA was present in 100%, pelA and ppyR in 99%; all isolates carried exoY and exoT, exoS was found in 88% and exoU in 27%. Multidrug resistance was 15%. Biofilm phenotype was unrelated to resistance. In contrast, levofloxacin resistance was more frequent among exoU-positive isolates (29.6% vs. 9.6%; p = 0.024), and this association survived adjustment for biofilm phenotype and hospital unit in multivariable analysis (adjusted OR 3.90, 95% CI 1.20–12.67). However, the association did not remain significant after correction for multiple comparisons or after excluding exoS/exoU co-positive isolates. The exoU–fluoroquinolone relationship is therefore presented as an exploratory, hypothesis-generating finding that requires confirmation by reference susceptibility testing and clonal characterisation.